1. Technical Field
This invention relates to a degassing module which is used as a degassing unit for high performance liquid chromatography (hereinafter called HPLC) systems. It removes gaseous compounds dissolved in eluent which is separated and analyzed by the liquid chromatography, thereby allowing to make a fine and precise fluid delivery at a high speed required for HPLC.
2. Background Art
There is a tendency that HPLC used to separate compounds in a given sample is made more and more highly accurate. Usually in this type of HPLC, an eluent drawn from a reservoir by a fluid delivery pump is delivered via a sample injection valve to a detecting section including a separation column. In high speed and high accuracy liquid chromatography systems (known as semi-micro HPLC and micro HPLC) which requires high accuracy in delivering the eluent under a high pressure yet at a very small quantity, it is common to install a degassing unit on the inlet side of the fluid delivery pump in order to insure the stability of the pump.
The purpose to install this type of degassing unit is to remove unnecessary gases dissolved in the eluent. Especially when an electrode reduction reaction is measured, oxygen dissolved in the eluent greatly influence its measured value. That is, the reduction reaction of the oxygen themselves makes a big background current, thereby causing noises to be increased.
FIG. 10 is a block diagram to explain a system configuration of HPLC. An eluent 2 in a first reservoir 1 is drawn up by a pump 5 through a pipe 3 and degassed by a degassing unit 4. It is then delivered through a sample injection valve (auto sampler) 6 and 3 column 7 to a detector unit 8. The eluent delivered from the detector unit 8 is thrown out to a second reservoir 10 as a waste eluent 9. The arrow marks show the direction of the eluent delivery. Data detected by the detector unit 8 are transferred to a data processing unit 11, wherein they are processed in a visual form or a computer processable data form to provide and store. The column 7 is accommodated in an isothermal oven 7A to prevent the influence of external temperature. The pump 5 and the sample injection valve 6 are controlled by a system controller 12. The degassing unit 4 is installed before the pump 5 to insure the stable delivery of the fluid and the accurate analysis by removing gases dissolved in the eluent which is drawn up from the first reservoir 1 by the pump 5.
As it is well known about other units and components consisting of this kind of high accuracy liquid chromatography as well as about the function of the whole system, those explanations are omitted.
FIGS. 11(a) and (b) illustrate a compositional example of a conventional degassing unit, showing its overall cross sectional view in FIG. 11(a) and its principal part in cross section in FIG. 11(b), respectively.
As shown in FIG. 11(a), the conventional degassing unit is composed of a degassing module 14 accommodated inside a hermetically sealed room (vacuum chamber) 13 and a vacuum pump 15 to evacuate air from the chamber 13. The degassing module 14 consists of capillary tubes 16 made of such material inactive to corrosive liquid or gases in various organic solvents as Polytetra Fluoro Ethylene (PTFE), both of which ends are bundled by multi-connectors 16a and 16b. The multi-connector 16a is connected through a joint tube 17a to the outside of the chamber 13. The joint tube 17a is then connected to the first reservoir 1 via the pipe 3 as shown in FIG. 10. The multi-connector 16b is connected through a joint tube 17b to the outside of the chamber 13 as well, where it is then connected to the inlet of the pump 5 as shown in FIG. 10. A number of the capillary tubes 16 made of gas permeable resin like PTFE or others to compose the degassing module 14 are bundled as shown in the FIG. 11(b). The inside of the chamber 13 is reduced in pressure by the vacuum pump 15, thereby reducing the pressure in the space between the bundled capillary tubes. Consequently, gases dissolved in the eluent flowing inside the capillary tubes 16 are extracted to the inside of the chamber 13.
As mentioned above, the conventional degassing unit is composed of the degassing module 14 made with the capillary tubes of gas permeable resin such as silicon resin or PTFE which is inactive to the eluent yet good in gas permeability, the vacuum chamber 13, and the vacuum pump 15. By flowing the eluent in the capillary tunes 16 of PTFE or others for the degassing module 14, and by reducing the pressure outside the capillary tubes by the vacuum pump 15, the degassing unit is provided with the function to remove gaseous compounds dissolved in the eluent flowing inside the capillary tubes. The eluent is degassed by passing through the degassing unit, and sent out to the detection means by the fluid delivery pump, thereby making it possible to deliver the eluent stably and accurately to the detection means regardless of changes in ambient temperature.
Generally, an amount of gases dissolved in a liquid is not always constant. It is proportional to pressure, and the higher the pressure the more the amount of gases dissolved. This means the amount of gases dissolved is dependent on a change of the atmospheric pressure.
On the other hand, it is also dependent on temperature. If the pressure is constant, the lower the temperature the more the amount of gases dissolved. To put it concretely, as the temperature is lower in the morning than in the day time, the amount of gases dissolved in the eluent is large in the morning and becomes small in the day time. When starting delivering the fluid in the morning, gas bubbles tend to be observed in the capillary tubes connected to the fluid pump as the temperature goes up toward the day time. It shows that the amount of gases dissolved in the eluent is reduced by an increase of the surrounding temperature. The generation of gas bubbles due to the increase of the temperature becomes a worst condition for such a case that the eluent has to be cooled.
For example, when the room temperature fluctuates in a range of 25-30 degrees C., the temperature of the eluent that is cooled at 5 degrees fluctuates at 20-25 degrees C. and it is observed that gas bubbles are suddenly generated inside the capillary tubes.
In this connection, the degassing unit plays an important role to perform the stable fluid delivery of fine amount precisely regardless of changes of the surrounding conditions. For details on this type of high accuracy liquid chromatography, refer to U.S. Pat. No. 5,472,598.
A standard high performance liquid chromatography (HPLC) system being conventionally used is configured with its column sizes of 4.6 mm in diameter and 250 mm in length at a speed (pump flow speed) of 1-1.5 ml/min. in fluid delivery by a pump.
In addition to this, recently developed were semi-micro HPLC systems with a very small volume of column by making its size smaller which is 1.0-2.0 mm in diameter and 250 mm in length at a fluid delivery speed of 50-300 .mu.l/min. Those were followed by micro HPLC systems by making its column size further smaller which is 0.5-1.0 mm in diameter and 250 mm in length at a fluid delivery speed of 10-50 .mu.l/min. In order to configure such a micro HPLC system as mentioned above, it is required to make an attached degassing unit being of microstructure as well.
However, in attempting the degassing unit to be of micro-structure problems arise in an internal volume and a structure of a degassing module as the functional part. The internal volume of a degassing module in a standard HPLC system has a capacity of 12 ml, which is too big to apply to the semi-micro and micro HPLC systems. That is, as the pump flow speed in semi-micro HPLC is 0.1-0.2 ml/min. as mentioned above, it takes 60-120 min. even in simple calculation for the fluid to pass through the degassing module. This is considerably long for the time of chromatographic analysis.
The capillary tubes of PTFE or others for the degassing module are commonly used in a bundle of 18 tubes with the length of 2,500 mm. The fluid is distributed into the respective capillary tubes to flow. However, as every tube is not the same in flow resistance, speeds for the fluid to pass through the tubes are different between them. Because of the difference in fluid speed, the time required for the fluid to be replaced completely inside the module is not 60-120 minutes as previously obtained in simple calculation, but it actually takes a few to several times more (325 minutes) as shown in Table 1.
Therefore, even if the volume of the tubes is made smaller in order to make the module internal volume small, it takes long to replace the fluid in the area of semi-micro HPLC if the structure of the degassing module is kept the same as it is.
Table 1 shows a relationship of fluid replacement volume and time by the conventional tube module in comparison with by flat film module of this invention which details are described hereunder.
TABLE 1 ______________________________________ Replacement Replacement Replacement time time volume (1 ml/min) (0.2 ml/min) ______________________________________ Tube module 65 ml 65 min. 325 min. Flat film module 9.8 ml 9.8 min. 49 min. ______________________________________ (Note) Internal volume of module Tube module: 12 ml Flat film module: 1.6 ml
In this connection, there was a problem that even if just applying the conventional degassing module to semi-micro HPLC it greatly increases the replacement time, making it unable to suitably comprise a semi-micro HPLC system of practical performance.